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Information from the Skies

Figure 3.1 shows a galaxy in the constellation Andromeda. On a dark, clear night, far from cities or other sources of light, the Andromeda Galaxy, as it is generally called, can be seen with the naked eye as a faint, fuzzy patch on the sky, comparable in diameter to the full Moon. Yet the fact that it is visible from Earth belies this galaxy's enormous distance from us. It lies roughly 2.5 million light-years away. An object at such a distance is truly inaccessible in any realistic human sense. Even if a space probe could miraculously travel at the speed of light, it would need 2.5 million years to reach this galaxy and 2.5 million more to return with its findings. Considering that civilization has existed on Earth for fewer than 10,000 years, and its prospects for the next 10,000 are far from certain, even this unattainable technological feat would not provide us with a practical means of exploring other galaxies. Even the farthest reaches of our own galaxy, "only" a few tens of thousands of light-years distant, are effectively off-limits to visitors from Earth, at least for the foreseeable future.

LIGHT AND RADIATION

Figure 3.1 Andromeda The pancake-shaped Andromeda Galaxy lies about 2.5 million light-years away, according to the most recent distance measurements. It contains a few hundred billion stars. (T. Hallas)

Given the impossibility of traveling to such remote parts of the universe, how do astronomers know anything about objects far from Earth? How do we obtain detailed information about any planet, star, or galaxy too distant for a personal visit or any kind of controlled experiment? The answer is that we use the laws of physics, as we know them here on Earth, to interpret the electromagnetic radiation emitted by these objects. Radiation is any way in which energy is transmitted through space from one point to another without the need for any physical connection between those two locations. The term electromagnetic just means that the energy is carried in the form of rapidly fluctuating electric and magnetic fields (to be discussed in more detail later in Section 3.2). Virtually all we know about the universe beyond Earth's atmosphere has been gleaned from painstaking analysis of electromagnetic radiation received from afar.

Visible light is the particular type of electromagnetic radiation to which our human eyes happen to be sensitive. As light enters our eye, the cornea and lens focus it onto the retina, whereupon small chemical reactions triggered by the incoming energy send electrical impulses to the brain, producing the sensation of sight. But there is also invisible electromagnetic radiation, which goes completely undetected by our eyes. Radio, infrared, and ultraviolet waves, as well as X-rays and gamma rays, all fall into this category. Recognize that, despite the different names, the words light, rays,radiation, and waves really refer to the same thing. The names are just historical accidents, reflecting the fact that it took many years for scientists to realize that these apparently very different types of radiation are in reality one and the same physical phenomenon. Throughout this text, we will use the general terms "light" and "electromagnetic radiation" more or less interchangeably.

WAVE MOTION

Despite the early confusion still reflected in current terminology, scientists now know that all types of electromagnetic radiation travel through space in the form of waves. To understand the behavior of light, then, we must know a little about wave motion.

Simply stated, a wave is a way in which energy is transferred from place to place without physical movement of material from one location to another. In wave motion, the energy is carried by a disturbance of some sort. This disturbance, whatever its nature, occurs in a distinctive repeating pattern. Ripples on the surface of a pond, sound waves in air, and electromagnetic waves in space, despite their many obvious differences, all share this basic defining property.

Imagine a twig floating in a pond. A pebble thrown into the pond at some distance from the twig disturbs the surface of the water, setting it into up-and-down motion. This disturbance will propagate outward from the point of impact in the form of waves. When the waves reach the twig, some of the pebble's energy will be imparted to it, causing the twig to bob up and down. In this way, both energy and information—the fact that the pebble entered the water—are transferred from the place where the pebble landed to the location of the twig. We could tell that a pebble (or, at least, some object) had entered the water just by observing the twig. With a little additional physics, we could even estimate the pebble's energy.

A wave is not a physical object. No water traveled from the point of impact of the pebble to the twig—at any location on the surface, the water surface simply moved up and down as the wave passed. What, then, did move across the surface of the pond? As illustrated in Figure 3.2, the answer is that the wave was the pattern of up-and-down motion. This pattern was transmitted from one point to the next as the disturbance moved across the water.

Figure 3.2 Water Wave The passage of a wave across a pond causes the surface of the water to bob up and down, but there is no movement of water from one part of the pond to another. Here waves ripple out from the point where a pebble has hit the water to the point where a twig is floating. The inset shows schematically a series of "snapshots" of part of the pond surface as the wave passes by. The points numbered 1 through 5 represent nearby particles on the surface.

Figure 3.3 shows how wave properties are quantified and illustrates some standard terminology.The wave period is the number of seconds needed for the wave to repeat itself at some point in space. The wavelength is the number of meters needed for the wave to repeat itself at a given moment in time. It can be measured as the distance between two adjacent wave crests, two adjacent wave troughs, or any other two similar points on adjacent wave cycles (for example, the points marked in the figure). The maximum departure of the wave from the undisturbed state—still air, say, or a flat pond surface—is called its amplitude.

The number of wave crests passing any given point per unit time is called the wave's frequency. If a wave of a given wavelength moves at high speed, then many crests pass per second and the frequency is high. Conversely, if the same wave moves slowly, then its frequency will be low. The frequency of a wave is just one divided by the wave's period:

Frequency is expressed in units of inverse time (cycles per second), called hertz (Hz) in honor of the nineteenth-century German scientist Heinrich Hertz, who studied the properties of radio waves. Thus a wave with a period of 5 s has a frequency of (1/5) cycles/s = 0.2 Hz, meaning that one wave crest passes a given point in space every five seconds.

A wave moves a distance equal to one wavelength in one wave period. The product of wavelength and frequency therefore equals the wave velocity:

wavelength x frequency = velocity.

Thus, if the wave in our earlier example had a wavelength of 0.5 m, its velocity is (0.5 m) x (0.2 Hz) = 0.1 m/s. Wavelength and wave frequency are inversely related—doubling one halves the other.

Before about 1800, scientists were divided in their opinions about the nature of light. Some believed that light was a wave phenomenon, while others maintained that light was in reality a stream of particles that moved in straight lines. Given the experimental apparatus available at the time, neither camp could find conclusive evidence to disprove the other theory. Discovery 3-1discusses some more wave properties of importance to modern astronomers, and describes how their detection in experiments using visible light early in the nineteenth century finally tilted the balance of scientific opinion in favor of the wave theory.

THE COMPONENTS OF VISIBLE LIGHT

White light is a mixture of colors, which we conventionally divide into six major hues—red, orange, yellow, green, blue, and violet. As shown in Figure 3.4, we can separate a beam of white light into a rainbow of these basic colors—called a spectrum (plural, spectra)—by passing it through a prism. This experiment was first reported by Isaac Newton over 300 years ago. In principle, the original beam of white light could be recovered by passing the spectrum through a second prism to recombine the colored beams.

What determines the color of a beam of light? The answer is its frequency (or equivalently, its wavelength). We see different colors because our eyes react differently to electromagnetic waves of different frequencies. A prism splits a beam of light up into separate colors because light rays of different frequencies are bent, or refracted, slightly differently as they pass through the prism—red light the least, violet light the most. Red light has a frequency of roughly 4.3 x 1014Hz, corresponding to a wavelength of about 7.0 x 10-7 m. Violet light, at the other end of the visible range, has nearly double the frequency—7.5 x 1014 Hz—and (since the speed of light is the same in either case) just over half the wavelength—4.0 x 10-7 m. The other colors we see have frequencies and wavelengths intermediate between these two extremes, spanning the entire visible spectrum shown in Figure 3.4. Radiation outside this range is invisible to human eyes.

Figure 3.3 Wave Properties Representation of a typical wave, showing its direction of motion, wavelength, and amplitude. In one wave period, the entire pattern shown here moves one wavelength to the right.

Scientists often use a unit called the nanometer (nm) when describing the wavelength of light (see Appendix 2). There are 109 nanometers in one meter. An older unit called the angstrom (1 Å = 10-10 m = 0.1 nm) is also widely used. (The unit is named after the nineteenth-century Swedish physicist Anders Ångstrom—pronounced "ong.strem.") However, in SI units, the nanometer is preferred. Thus, the visible spectrum covers the wavelength range from 400 nm to 700 nm (4000 Å to 7000 Å). The radiation to which our eyes are most sensitive has a wavelength near the middle of this range, at about 550 nm (5500 Å), in the yellow-green region of the spectrum. It is no coincidence that this wavelength falls within the range of wavelengths at which the Sun emits most of its electromagnetic energy—our eyes have evolved to take greatest advantage of the available light.

Figure 3.4 Visible Spectrum While passing through a prism, white light splits into its component colors, spanning red to violet in the visible part of the electromagnetic spectrum. The slit narrows the beam of radiation. The image on the screen is just a series of different-colored images of the slit. Human eyes are insensitive to radiation of wavelength shorter than 400 nm or longer than 700 nm, but radiation outside the visible range is easily detected by other means.